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The substituent effects of naphthalene diimide as acceptor for organic solar cells: A theoretical study
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The substituent effects of naphthalene diimide as acceptor for organic solar cells: A theoretical study Shanshan Tang a,b, Xiaoli Lv b, Dan Liu c, Zhuoxin Li b, Songyang Li b, Guang Chen a,∗, Lijuan Kang b,∗∗, Dadong Liang b,∗∗, Ruifa Jin d a
College of Life Sciences, Jilin Agricultural University, Changchun, Jilin 130118, China College of Resource and Environmental Science, Jilin Agricultural University, Changchun 130118, China Changchun Institute of Measurement and Testing Technology, Changchun, Jilin, China d College of Chemistry and Chemical Engineering, Chifeng University, Chifeng 024000, China b c
a r t i c l e
i n f o
Article history: Received 20 October 2016 Revised 20 March 2017 Accepted 10 April 2017 Available online xxx Keywords: Substituent effects Naphthalene diimide Acceptor Organic solar cells Theoretical study
a b s t r a c t The substituent effects of naphthalene diimide (NDI) have been investigated by employing the CAMB3LYP/6-31G(d) and TD-B3LYP/6-31+G(d,p) methods in order to design proper acceptor of solar cell with excellent performances, for example, the suitable frontier molecular orbital (FMO) energies to match those of oligo(thienylenevinylene) derivatives. The simulated results show that the different substituents significantly affect the distribution patterns of FMOs for NDI. The pull substituents could decrease the FMO energies and energy gap of NDI. Introducing the proper substituents to molecule NDI could make their FMOs being suitable for oligo(thienylenevinylene) derivatives. The different substituents significantly affect the absorption spectra of NDI. The electron withdrawing group and/or electron donating group substituents can improve the electron transfer properties of NDI. © 2017 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
1. Introduction Since the organic solar cells (OSCs) appeared, owing to its lightweight, mechanical flexibility, potential of low cost fabrication, and rapid energy payback time [1], it received worldwide attention in both academia and industry [2]. Now the OSCs mainly utilize fullerene and its derivatives, such as [6,6]-phenyl-C61 (or C71 )-butyric acid methyl ester (PC61 BM or PC71 BM), as the electron acceptor materials. Fullerene and its derivatives are good electron acceptors and electron transporting materials. Nevertheless, it is difficult to modify the backbone of fullerene and its derivatives chemically. Thus, the absorption region and the frontier molecular orbit (FMO) energy levels cannot be readily tuned [3]. Recently, more and more researchers pay attention to the organic small molecules on the basis of the π -conjugate system as the acceptor materials of the OSCs, because of their various molecular structures, easily tunable absorption region and FMO energy levels, low cost, and light weight [4]. Among these organic acceptors, naphthalene diimide (NDI) based on the conjugated polymer has ∗ Corresponding author at: College of Life Sciences, Jilin Agricultural University, Changchun, Jilin 130118, China. ∗∗ Corresponding authors. E-mail addresses: [email protected], [email protected] (G. Chen), [email protected] (D. Liang).
been recognized as the most effective acceptor material [5–7]. Zhou et al. investigated the active layer photovoltaic performance, charge transport, and blend morphology of the all-polymer OSCs construct of p-type PTPD3T and n-type N2200 (NDI derivative) [8]. Their work not only could highlight the importance of molecular weight tuning for both polymer components, but also could provide a promising strategy and relevant synthetic methodologies to utilize the conjugated polymers with desired molecular weight for optimizing future efficiencies of all-polymer solar cells. Zhou et al. fabricated the OSCs on the basis of the copolymer of NDI and bithiophene as the near-infrared absorber and electron acceptor, and PTB7 was as the electron donor [9]. The external quantum efficiency spectra of the OSCs displayed photoresponse up to 900 nm with the efficiency of 25% at 800 nm. Earmme et al. provided an investigation of three NDI copolymers as electron acceptors in OSCs. They found that the highest power conversion efficiency (PCE) of OSCs on the basis of the NDI copolymer as acceptor and the thiazolothiazole copolymer as donor is 3.3% [10]. Its short circuit current density and external quantum efficiency are 7.78 mA/cm2 and 47%. Lee et al. produced a highly efficient OSC device with the PCE of 5.96% by taking a series of naphthalene diimide-based polymer as the acceptors [11]. Hwang et al. achieved an OSC device with the PCE of 7.7% utilizing the naphthalene diimide–selenophene copolymer as acceptor and
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Please cite this article as: S. Tang et al., The substituent effects of naphthalene diimide as acceptor for organic solar cells: A theoretical study, Journal of the Taiwan Institute of Chemical Engineers (2017), http://dx.doi.org/10.1016/j.jtice.2017.04.015
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O
R1
R2
H3C N O
Fig. 1. The structures of donors X1 and X2.
benzodithiophene–thieno[3,4-b]thiophene copolymer as donor [12]. Gao et al. prepared OSCs with a high PCE of 8.27% and a high fill factor of 70.24% using N2200 as acceptor and J51 as donor [13]. Hwang et al. designed a series of NDI/perylene diimide (PDI)–selenophene random copolymers, and investigated them as acceptors in OSCs [14]. They obtained the fine performances with the highest PCE of 6.3% and external quantum efficiency of 91%. Sakai et al. reviewed researches of the core-substituted naphthalene diimides [15] and investigated the electron- and hole-transporting pathways of naphthalene diimide derivatives [16]. Yushchenko et al. gave the comparison of charge-transfer dynamics of naphthalene diimide triads in solution [17]. Monti et al. investigated a series of donor–antenna–acceptor molecular rectifiers designed as modules for artificial photosynthesis devices with the naphthalene diimides as the antenna and secondary electron acceptor [18]. They also proposed a photoanode in which a TiO2 substrate is functionalized with the naphthalene diimide dye [19]. In the present work, we investigated a series of NDI derivatives with different substituent groups as OSCs acceptors. The purpose of introducing substituent groups is to improve the performances of NDI, such as the proper FMO energy levels matching those of the donors oligo(thienylenevinylene) derivatives (X1 and X2, Fig. 1) with excellent performances investigated by Yong and Zhang before [20], broad absorption region, and high charge transfer rate. Molecules X1 and X2 are the donor molecules in the study of Yong and Zhang with excellent properties, such as superior absorption properties. The difference between them is that the position of atom N in the substituent group connected with the thiophene is different. It is well known that, increasing the lowest unoccupied molecular orbital (LUMO) energy of the acceptor could increase the open circuit voltage (Voc ), because the Voc value increases along with the difference between the highest occupied molecular orbital (HOMO) energy of the donor and LUMO energy of the acceptor increasing. Additionally, the LUMO energy of the donor should be higher than that of acceptor larger than 0.30 eV The substituent groups could affect the molecular properties significantly, thus three kinds of molecules (NDI1-22, Scheme 1) were designed to study the push (–OCH3 and –C6 H6 ), pull (–CN and –NO2 ), and push–pull (–CN and –OCH3 as well as –C6 H6 and –NO2 ) substituent groups effects. We investigated the ground state properties of these molecules by using the density functional theory (DFT) [21], for example HOMO energy, LUMO energy, and HOMO–LUMO gap (Eg ). The absorption spectra of the designed molecules were evaluated by the time dependent DFT [22–24] approach (TD-DFT). We also simulated the charge transfer properties (reorganization energy, λ). Moreover, the correlation between structures and properties of the designed molecules were discussed.
O N CH3
R3
NDI1: R1 = -CN NDI2: R1 = R2 = -CN NDI3: R1 = R3 = -CN NDI4: R1 = R4 = -CN NDI5: R1 = -OCH3 NDI6: R1 = R2 = -OCH3 NDI7: R1 = R3 = -OCH3 NDI8: R1 = R4 = -OCH3 NDI9: R1 = -CN, R2 = -OCH3 NDI10: R1 = -CN, R3 = -OCH3 NDI11: R1 = -CN, R4 = -OCH3
R4
O
NDI12: R1 = -C6H6 NDI13: R1 = R2 = -C6H6 NDI14: R1 = R3 = -C6H6 NDI15: R1 = R4 = -C6H6 NDI16: R1 = -NO2 NDI17: R1 = R2 = -NO2 NDI18: R1 = R3 = -NO2 NDI19: R1 = R4 = -NO2 NDI20: R1 = -C6H6, R2 = -NO2 NDI21: R1 = -C6H6, R3 = -NO2 NDI22: R1 = -C6H6, R4 = -NO2
Scheme 1. Chemical structures of NDI derivatives. Table 1 The predicted EHOMO , ELUMO , and Eg values of NDI and its derivatives at the CAM-B3LYP/6-31G(d) level.
NDI NDI1 NDI2 NDI3 NDI4 NDI5 NDI6 NDI7 NDI8 NDI9 NDI10 NDI11 NDI12 NDI13 NDI14 NDI15 NDI16 NDI17 NDI18 NDI19 NDI20 NDI21 NDI22
EHOMO (eV)
ELUMO (eV)
Eg (eV)
−8.34 −8.74 −9.05 −9.12 −9.10 −7.90 −8.19 −7.71 −7.45 −8.60 −8.28 −8.24 −7.98 −7.70 −7.96 −7.83 −8.78 −9.13 −9.21 −9.18 −8.25 −8.31 −8.22
−2.28 −2.74 −3.09 −3.16 −3.18 −2.12 −2.11 −1.98 −1.99 −2.65 −2.57 −2.56 −2.22 −2.14 −2.15 −2.17 −2.70 −3.07 −3.11 −3.11 −2.57 −2.62 −2.63
6.07 6.00 5.96 5.96 5.92 5.77 6.09 5.73 5.46 5.95 5.71 5.68 5.76 5.56 5.81 5.66 6.08 6.06 6.10 6.07 5.68 5.70 5.60
2. Computational details Recently, DFT method attracts extensive attention in OSCs field worldwide because of its application in explanation and predicting the properties of OSCs [25,26]. The DFT method CAM-B3LYP/631G(d,p) was proved to be reasonable for optimization of perylene diimide (PDI) and its derivatives, and the TD-B3LYP/6-31+G(d,p) method reliable for optical property simulation by our previous works [27,28]. Moreover, the chemical structure of NDI (Scheme S1) and the calculated results (Table S1) are listed in the supporting information. It could be seen that the CAM-B3LYP method was reliable in comparison with the crystal data [29]. Consequently, we employed the CAM-B3LYP/6-31G(d,p) method to optimize all the geometry parameters including neutral, cation, and anion of molecules NDI1-22 with no symmetry. The B3LYP/6-31+G(d,p) method was used to predict the absorption spectra of molecules NDI1-22. There is no imaginary frequency in the calculations. The PBE1PBE/6-31G(d) method was used to optimize the geometry of molecules X1 and X2 [20], and the HOMO and LUMO energies of molecules X1 and X2 were calculated at the CAM-B3LYP/6-
Please cite this article as: S. Tang et al., The substituent effects of naphthalene diimide as acceptor for organic solar cells: A theoretical study, Journal of the Taiwan Institute of Chemical Engineers (2017), http://dx.doi.org/10.1016/j.jtice.2017.04.015
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31G(d,p) level on the basis of the single point energy. The CAMB3LYP/6-31G(d,p) single point calculations were performed according to the CAM-B3LYP/6-31G(d,p) optimized neutral, cationic, and anionic geometries. The reorganization energy is composed of two parts, external reorganization energy (λext ) and internal reorganization energy (λint ). λext reflects the effect of polarized medium on
3
charge transfer. λint represents the structural change between ionic and neutral states [30]. For the solid-state film of OSC materials, its low dielectric constant of medium leads to λext being not the main factor of λ [31]. Hence, we mainly discussed the λint of the isolated active organic π -conjugated systems owning to ignoring any environmental relaxation and changes in the present work. As
Fig. 2. The distribution patterns of FMO for NDI and its derivatives.
Please cite this article as: S. Tang et al., The substituent effects of naphthalene diimide as acceptor for organic solar cells: A theoretical study, Journal of the Taiwan Institute of Chemical Engineers (2017), http://dx.doi.org/10.1016/j.jtice.2017.04.015
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Fig. 2. Continued
Fig. 3. The EHOMO and ELUMO values of FMOs for NDI and its derivatives.
where E0 + (E0 − ) means the energy of the cation (anion) obtained by optimizing structure of the neutral molecule. Similarly, E+ (E− ) represents the energy of the cation (anion) obtained on the basis of the optimized cation (anion) structure, and E+ 0 (E− 0 ) is the single point energy of the neutral molecule simulated at the cationic (anionic) state. Finally, E0 is the single point energy of the neutral molecule calculated at the ground state. All the calculations were performed with the aid of Gaussian 09 software [32]. 3. Results and discussion 3.1. Frontier molecular orbitals
Fig. 4. The simulated HOMO and LUMO energies of NDI2-4 and NDI17-19 at the CAM-B3LYP/6-31G(d,p) level as well as the HOMO and LUMO energies of X1 and X2 at the CAM-B3LYP/6-31G(d,p)//PBE1PBE/6-31G(d) level.
a result, Eqs. (1) and (2) can be used for simulating the electron reorganization energy (λe ) and hole reorganization energy (λh ) values [31].
λe = E0 − − E− + E− 0 − E0
(1)
λh = E0 + − E+ + E+ 0 − E0
(2)
The distribution patterns of the FMO are important for characterizing the optical and electronic properties of molecules. The electron spatial distributions of HOMO and LUMO orbitals of NDI and its derivatives are plotted in Fig. 2. As shown in Fig. 2, for molecules NDI and NDI1-11, the distribution patterns of FMOs are spread over the whole molecules. It reveals that the –CN and –OCH3 groups with different amount and positions affect the FMO distributions of NDI slightly. For –C6 H6 group substituent molecules, the HOMO patterns of NDI12 and NDI15 are distributed on the NDI molecules and the –C6 H6 groups. Their LUMO patterns are delocalized on the NDI molecules. For molecules NDI13 and NDI14, their HOMO electronic density contours are distributed on their –C6 H6 groups, and their LUMO electronic density contours delocalized on the NDI molecules. These results show that the amount and positions of –C6 H6 groups affect the electronic density contours of FMOs for NDI significantly. For
Please cite this article as: S. Tang et al., The substituent effects of naphthalene diimide as acceptor for organic solar cells: A theoretical study, Journal of the Taiwan Institute of Chemical Engineers (2017), http://dx.doi.org/10.1016/j.jtice.2017.04.015
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Table 2 Calculated R (nm), λmax (nm), λmin (nm), and corresponding f values of NDI and its derivatives (H = HOMO, L = LUMO, L + 1 = LUMO + 1, etc.).
λmax
f
Composition
λmin
f
Composition
R
NDI
366.65
0.30
H → L (+70%)
219.71
0.06
146.94
NDI1
371.45
0.25
H → L (+70%)
233.28
0.08
NDI2
377.56
0.20
H → L (+70%)
252.41
0.13
NDI3
374.02
0.21
H → L (+70%)
246.99
0.24
NDI4
376.11
0.21
H → L (+70%)
255.69
0.56
NDI5
403.66
0.20
H → L (+69%)
228.03
0.06
NDI6
363.88
0.26
H-2 → L (−18%) H → L (+67%)
230.63
0.02
NDI7
406.31
0.20
H → L (+70%)
238.54
0.04
NDI8 NDI9
435.71 395.18
0.21 0.01
H → L (+70%) H-3 → L (+61%) H-2 → L (−21%) H-1 → L (+11%) H → L (−22%)
225.85 238.73
0.17 0.01
NDI10
413.54
0.18
H → L (+69%)
236.31
0.04
NDI11
411.72
0.19
H → L (+69%)
240.53
0.02
NDI12
468.78
0.04
H → L (+69%)
236.48
0.16
NDI13
363.62
0.27
H-4 → L (+70%)
306.51
0.17
NDI14
363.60
0.27
H-4 → L (+70%)
305.66
0.17
NDI15
474.29
0.08
H → L (+69%)
269.78
0.33
NDI16
367.88
0.23
266.44
0.02
NDI17
379.01
0.01
H-2 → L (+12%) H → L (+69%) H-4 → L (+24%) H-3 → L (+62%) H → L (−20%)
271.86
0.02
H-4 → L + 1 (+69%) H-2 → L + 1 (−14%) H-5 → L + 1 (−33%) H-4 → L + 1 (+42%) H-2 → L + 1 (+31%) H-2 → L + 2 (−27%) H → L + 1 (+10%) H-8 → L (+23%) H-5 → L + 1 (−15%) H-5 → L + 3 (+10%) H-2 → L + 1 (+46%) H → L + 2 (+42%) H-9 → L (−12%) H-8 → L (−23%) H-4 → L + 3 (−16%) H-2 → L (+12%) H → L + 1 (−14%) H → L + 2 (+58%) H-5 → L (−19%) H-5 → L + 1 (−11%) H-5 → L + 3 (+13%) H-3 → L (+16%) H-3 → L + 1 (−11%) H → L + 1 (+61%) H-3 → L + 1 (+67%) H → L + 3 (−16%) H-11 → L (−14%) H-5 → L + 1 (+11%) H-7 → L + 1 (−19%) H-5 → L + 1 (−17%) H-4 → L + 3 (−11%) H-2 → L + 2 (−35%) H-2 → L + 3 (+27%) H → L + 3 (+37%) H-3 → L + 1 (+65%) H → L + 2 (+11%) H → L + 3 (−21%) H-4 → L + 1 (+68%) H-7 → L + 1 (+10%) H-6 → L + 1 (+12%) H-4 → L + 1 (+20%) H-3 → L + 2 (+54%) H-2 → L + 1 (+14%) H-2 → L + 2 (−26%) H-1 → L + 3 (+12%) H-3 → L + 1 (+65%) H-2 → L + 1 (−15%) H-2 → L + 2 (+12%) H-3 → L + 1 (+65%) H-2 → L + 2 (−19%) H-11 → L (+12%) H-5 → L + 1 (−20%) H-2 → L + 2 (+47%) H-2 → L + 3 (+33%) H-1 → L + 2 (−12%) H-1 → L + 3 (−12%) H-9 → L (+68%) H-4 → L+2 (−18%) H-9 → L (+66%) H-7 → L (+14%) H-4 → L + 2 (−18%) H-1 → L + 3 (+17%) H → L + 2 (+65%) H-11 → L (+49%) H → L + 2 (−48%) H-13 → L (+16%) H-12 → L + 1 (−25%) H-7 → L + 2 (−25%) H-7 → L + 3 (+14%) H-6 → L + 1 (−11%) H-5 → L + 2 (+11%) H-3 → L + 1 (+13%) H-1 → L + 2 (−24%) H-1 → L + 3 (+12%) H → L + 1 (+38%)
138.17
125.15
127.03
120.42
175.63 133.25
167.77
209.86 156.45
177.23
171.19 232.30
57.11 57.94
204.51 101.44 107.15
(continued on next page)
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S. Tang et al. / Journal of the Taiwan Institute of Chemical Engineers 000 (2017) 1–9 Table 2 (continued)
λmax
f
Composition
λmin
f
Composition
R
NDI18
366.53
0.20
297.94
0.07
H-7 → L (+70%)
68.59
NDI19
375.83
0.14
H-1 → L (−13%) H → L (+68%) H-1 → L (+25%) H → L (+65%)
263.21
0.05
112.62
NDI20
368.74
0.22
320.43
0.13
NDI21
366.49
0.22
H-3 → L H-2 → L H-3 → L H-2 → L
279.91
0.01
NDI22
491.88
0.04
276.12
0.12
H-14 → L (+60%) H-1 → L + 4 (−11%) H → L + 4 (+32%) H-7 → L (+69%) H-2 → L + 3 (+13%) H-11 → L (+64%) H-10 → L + 1 (−10%) H-2 → L + 2 (+14%) H-2 → L + 3 (−16%) H-11 → L (+30%) H-2 → L + 3 (−13%) H → L + 3 (+56%) H → L + 4 (+19%)
(+17%) (+68%) (+16%) (+68%)
H → L (+69%)
molecules NDI6-19, the distribution patterns of FMOs are spread over the whole molecules. It displays that the –NO2 groups with different amount and positions affect the FMO distributions of NDI slightly. For molecules NDI20 and NDI21, their HOMO electronic density contours are distributed on the –C6 H6 groups, and their LUMO electronic density contours delocalized on the –NO2 groups and NDI molecules. For molecule NDI21, its HOMO pattern is nearly spread over the whole molecule, and its LUMO pattern is distributed on the –NO2 group and NDI molecule. In conclusion, the –CN and –OCH3 substituents affect the FMO patterns of NDI molecule slightly, and the –C6 H6 and –NO2 substituents affect them significantly. As is well-known, the FMO energies are very important for the spectral properties of the designed molecules, and they also relate to the reactivity of a compound. The evaluations of HOMO and LUMO energies (EHOMO and ELUMO ) for the investigated molecules are listed in Table 1 and plotted in Fig. 3. As shown in Table 1 and Fig. 3, one can find that introducing the –CN group to molecule NDI could decrease the EHOMO , ELUMO , and Eg values of NDI. The amount of decrease for EHOMO and ELUMO values are in the order of NDI4 > NDI3 > NDI2 > NDI1. Their Eg values are in the sequence of NDI4 < NDI3 = NDI2 < NDI1 < NDI. It reveals that –CN substituent can decrease the EHOMO , ELUMO , and Eg values of NDI, and di-CN substituents make larger decrease of EHOMO , ELUMO , and Eg values of NDI than the single –CN substituent does. For molecules NDI5-8, the –OCH3 group could increase the EHOMO and ELUMO values and decrease the Eg values of NDI, except the –OCH3 groups in 1,2-positions increase the Eg value of NDI. The increase of the EHOMO value is the largest when the –OCH3 group in the 4-position of NDI. The increase of the ELUMO value is the largest when the –OCH3 group is in the 3-position of NDI. The Eg value is the smallest when the –OCH3 group in the 4-position of NDI. For –CN and –OCH3 substituent molecules, their EHOMO values are in the order of NDI11 > NDI10 > NDI > NDI9, ELUMO values are in the sequence of NDI11 > NDI10 > NDI9 > NDI, and Eg values are in the order of NDI11 < NDI10 < NDI9 < NDI. These results display that the push–pull substituents influence the EHOMO , ELUMO , and Eg values of NDI significantly, and decrease the Eg values of NDI. For molecules NDI12-15, the –C6 H6 group could increase the EHOMO and ELUMO values and decrease the Eg values of NDI. The increases of the EHOMO and ELUMO values are the largest when the –C6 H6 group is in the 2-position of NDI, respectively. The Eg value is the smallest when the –C6 H6 group is in the 4-position of NDI. For –NO2 substituent molecules, their EHOMO values are in the order of NDI > NDI16 > NDI17 > NDI19 > NDI18, ELUMO values are in the sequence of NDI > NDI16 > NDI17 > NDI18 = NDI19, and Eg values are in the order of NDI18 > NDI16 > NDI = NDI19 > NDI17. These results indicate that –NO2 substituent could
48.31 86.58
215.76
decrease the EHOMO and ELUMO values of NDI obviously, and affect the Eg values of NDI slightly. For molecules NDI20-22, the –C6 H6 and –NO2 substituents could influence the EHOMO values slightly, and decrease the ELUMO and Eg values obviously. In conclusion, the pull substituents could decrease the EHOMO , ELUMO , and Eg values of NDI. The EHOMO and ELUMO values of molecules X1, X2, NDI2-4, and NDI17-19 are plotted in the Fig. 4. From Fig. 4, one can see that the ELUMO values of NDI2-4 and NDI17-19 are lower than those of X1 (0.41, 0.48, 0.50, 0.39, 0.43, and 0.43 eV) and X2 (0.39, 0.46, 0.48, 0.37, 0.41, and 0.41 eV), which indicates that NDI2-4 and NDI1719 are suitable acceptors for the FMOs of X1 and X2, respectively. These results show that the di-CN and di-NO2 sbstituents can decrease the FMOs of NDI. Therefore, introducing the proper substituents to molecule NDI could make their FMOs being suitable for molecules X1 and X2. 3.2. Absorption spectra The absorption region (R), the longest wavelengths of the absorption spectra (λmax ), the shortest wavelengths of the absorption spectra (λmin ), and the corresponding oscillator strength (f) of designed molecules are listed in Table 2. The simulated absorption spectra of designed molecules (considering the first 20 excited states) are shown in Fig. 5. The simulated absorption spectra of designed molecules were plotted by using the GaussSum 1.0 program [33]. As shown in Table 2 and Fig. 5, for molecules NDI1-5, the –CN substituent in different positions could increase the λabs-max and λabs-min values, and decrease the R values of NDI, respectively. Moreover, the amounts of R values decreased for di-CN substituent molecules are larger than that of single –CN substituent molecule. The calculated λabs-max value of molecule NDI4 is 376.11 nm, and the experiment datum of this is 380 nm [15]. It reveals that the computational methods are reasonable. For –OCH3 substituent molecules, the λabs-max values are in the increasing order NDI6 < NDI < NDI5 < NDI7 < NDI8, the λabs-min values are in the decreasing sequence NDI > NDI8 > NDI5 > NDI6 > NDI7, and the R values are in the order NDI6 < NDI < NDI7 < NDI5 < NDI8. It shows that the –OCH3 substituent can increase the λabs-max , λabs-min , and R values of NDI, except the diOCH3 substituent in 1,2-positions decrease the λabs-max and R values of NDI. In addition, the di-OCH3 substituent in 1,4-positions could make larger increase of R value than the other positions for NDI. For molecules NDI9-11, the –CN and –OCH3 substituents can increase the λabs-max , λabs-min , and R values of molecule NDI, and the di-CN–OCH3 substituents in 1,3-positions could make the largest increase of R value for NDI. For –C6 H6 substituent
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Fig. 5. The calculated absorption spectra of NDI and its derivatives (value of full width at half maximum is 30 0 0 cm−1 ).
molecules, the λabs-max values are in the order NDI15 > NDI12 > NDI > NDI13 ≈ NDI14, the λabs-min values are in the sequence NDI < NDI12 < NDI15 < NDI13 ≈ NDI14, and the R values are in the order NDI13 ≈ NDI14 < NDI < NDI15 < NDI12. It indicates that the –C6 H6 substituent in 1-position and 1,4-positions can increase
the R value, and the –C6 H6 substituent in 1,2-positions and 1,3positions can decrease the R value of molecule NDI. For molecules NDI16-19, the –NO2 substituent affects the λabs-max values slightly, increases the λabs-min values obviously, and decreases the R values significantly of molecule NDI. For –C6 H6 and –NO2 substituent
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Fig. 5. Continued
molecules, the λabs-max values are in the order NDI ≈ NDI21 < NDI20 < NDI22, the λabs-min values are in the decreasing sequence NDI20 > NDI21 > NDI22 > NDI, and the R values are in the increase order NDI20 < NDI21 < NDI < NDI22. It reveals that –C6 H6 and –NO2 substituents in 1,2- and 1,3-positions can affect the λabs-max values slightly, increase the λabs-min values obviously, and decrease the R values significantly of molecule NDI. The –C6 H6 and –NO2 substituents in 1,4-positions can increase the λabs-max , λabs-min , and R values significantly of molecule NDI. In conclusion, the λmax value of NDI22 is the largest and the R value of NDI12 is the largest among the designed molecules, indicating that they could be good potential candidates for the solar cell acceptors. 3.3. Charge transport property The charge transport property of material is important to design the acceptor for solar cell device, and the λ plays a role in charge transport and charge separation [34–39]. Understanding the relationship between molecular structure and charge transport property is important for designing good acceptors of solar cell devices. It is well known that the lower the reorganization energy, the higher the charge transfer rate [40,41]. We calculated the λ values of designed molecules on the basis of Eqs. (1) and (2). The calculated results are summarized in Table 3. As shown in Table 3, for molecules NDI1-4, the –CN substituent could decrease the λe values and increase the λh values of molecule NDI, except the –CN substituent in 1,3-positions could decrease the λh value of molecule NDI. It reveals that –CN substituent could improve the electron transfer rate of molecule NDI. For molecules NDI5-8, their λe and λh values are larger than those of molecule NDI, respectively. It indicates that –OCH3 substituent could decrease the charger transfer rate of molecule NDI. The λe values of molecules NDI9-11 are smaller than that of molecule NDI, and their λh values are larger than that of molecule NDI. It shows that –CN and –OCH3 substituents could increase the electron transfer property and decrease the hole transfer property of molecule NDI. For molecules NDI12-15, their λe and λh values are larger than those of molecule NDI, respectively, except the λe values of NDI12, NDI13, and NDI13 are smaller than that of molecule NDI. It displays that –C6 H6 substituent in 1-, 1,2- and 1,4-positions could increase the electron transfer rate of NDI. For molecules NDI16-22, their λe and λh values are larger than those of molecule NDI, respectively. It indicates that –NO2 as well as
Table 3 Calculated λe and λh (eV) values of NDI and its derivatives. CAM-B3LYP/6-31G(d,p)
NDI NDI1 NDI2 NDI3 NDI4 NDI5 NDI6 NDI7 NDI8 NDI9 NDI10 NDI11 NDI12 NDI13 NDI14 NDI15 NDI16 NDI17 NDI18 NDI19 NDI20 NDI21 NDI22
λe
λh
0.435 0.403 0.387 0.376 0.367 0.457 0.502 0.476 1.853 0.433 0.421 0.428 0.426 0.428 0.447 0.417 0.445 0.454 0.448 0.448 0.437 0.444 0.455
0.216 1.118 0.215 0.283 0.217 0.394 0.344 0.619 1.870 0.792 0.391 0.374 1.217 0.577 1.197 0.528 0.286 0.302 0.285 0.291 1.273 1.067 1.018
–C6 H6 and –NO2 substituents could decrease the charger transfer rate of molecule NDI. In conclusion, among these molecules, NDI4 has the best electron transport property, and NDI2 has the best hole transport property. 4. Conclusion In this work, we simulated the properties of the NDI and its derivatives. The calculated results revealed that the –CN and –OCH3 substituents affect the FMO patterns of NDI molecule slightly, and the –C6 H6 and –NO2 substituents affect them significantly. The –CN substituent can decrease the EHOMO , ELUMO , and Eg values of NDI. The –OCH3 group could increase the EHOMO and ELUMO values. The –C6 H6 group could increase the EHOMO and ELUMO values and decrease the Eg values of NDI. The –NO2 substituent could decrease the EHOMO and ELUMO values of NDI obviously, and affect the Eg values of NDI slightly. The different substituents in different positions could affect the λabs-max ,
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λabs-min , and R values significantly. The λmax value of NDI22 is the largest and the R value of NDI12 is the largest among the designed molecules. The –CN substituent could improve the electron transfer rate of molecule NDI. The –OCH3 substituent could decrease the charger transfer rate of molecule NDI. The –NO2 substituent could decrease the charger transfer rate of molecule NDI. Among these molecules, NDI4 has the best electron transport property, and NDI2 has the best hole transport property. Molecules NDI24 and NDI17-19 are suitable acceptors for X1 and X2, respectively. This study should be helpful in further theoretical and experimental study of such system on solar cell acceptor materials. Acknowledgments This work was financially supported by the Science and Technology Development Program Project of Jilin Province (No. 20170520145JH), the High-end Technology Innovation Platform of Straw Comprehensive Utilization Technology of College and University in Jilin Province (No. (2014)C-1), the Scientific Research Plan Project of the Education Department of Jilin Province (No. 2016170), and the National Natural Science Foundation of China (Nos. 21302062 and 21563002). Supplementary materials Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.jtice.2017.04.015. References [1] Darling SB, You F. The case for organic photovoltaics. RSC Adv 2013;3:17633–48. [2] Pelzera KM, Darling SB. Charge generation in organic photovoltaics: a review of theory and computation. Mol Syst Des Eng 2016;1:10–24. [3] He Y, Li Y. Fullerene derivative acceptors for high performance polymer solar cells. Phys Chem Chem Phys 2011;13:1970–83. [4] Pelzera KM, Darling SB. Charge generation in organic photovoltaics: a review of theory and computation. Mol Syst Des Eng 2016;1(1):10–24. [5] Zhan C, Yao J. More than conformational “twisting” or “coplanarity”: molecular strategies for designing high-efficiency nonfullerene organic solar cells. Chem Mater 2016;28:1948–64. [6] Tang XQ , Liu XR, Shen W, Hu WX, He RX, Li M. Theoretical investigations of the small molecular acceptor materials based on oligothiophene-naphthalene diimide in organic solar cells. RSC Adv 2016;6:102159–71. [7] Xue LW, Yang YK, Zhang ZG, Dong XN, Gao L, Bin HJ, et al. Indacenodithienothiophene-naphthalene diimide copolymer as an acceptor for all-polymer solar cells. J Mater Chem A 2016;4:5810–16. [8] Zhou NJ, Dudnik AS, Li TING, Manley EF, Aldrich TJ, Guo PJ, et al. All-polymer solar cell performance optimized via systematic molecular weight tuning of both donor and acceptor polymers. J Am Chem Soc 2016;138:1240–51. [9] Zhou EJ, Nakano M, Izawa S, Cong JZ, Osaka I, Takimiya K, et al. All-polymer solar cell with high near-infrared response based on a naphthodithiophene diimide (NDTI) copolymer. ACS Macro Lett 2014;3:872–5. [10] Earmme T, Hwang YJ, Murari NM, Subramaniyan S, Jenekhe SA. All-polymer solar cells with 3.3% efficiency based on naphthalene diimide-selenophene copolymer acceptor. J Am Chem Soc 2013;135:14960–3. [11] Lee C, Kang H, Lee W, Kim T, Kim KH, Woo HY, et al. High-performance all-polymer solar cells via side-chain engineering of the polymer acceptor: the importance of the polymer packing structure and the nanoscale blend morphology. Adv Mater 2015;27:2466–71. [12] Hwang YJ, Courtright BAE, Ferreira AS, Tolbert SH, Jenekhe SA. 7.7% efficient all-polymer solar cells. Adv Mater 2015;27:4578–84. [13] Gao L, Zhang ZG, Xue L, Min J, Zhang J, Wei Z. All-polymer solar cells based on absorption-complementary polymer donor and acceptor with high power conversion efficiency of 8.27%. Adv Mater 2016;28:1884–90. [14] Hwang YJ, Earmme T, Courtright BAE, Eberle FN, Jenekhe SA. n-type semiconducting naphthalene diimide-perylene diimide copolymers: controlling crystallinity, blend morphology, and compatibility toward high-performance all-polymer solar cells. J Am Chem Soc 2015;137:4424–34. [15] Sakai N, Mareda J, Vauthey E, Matile S. Core-substituted naphthalenediimides. Chem Commun 2010;46:4225–37.
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The substituent effects of naphthalene diimide as acceptor for organic solar cells: A theoretical study Shanshan Tang a,b, Xiaoli Lv b, Dan Liu c, Zhuoxin Li b, Songyang Li b, Guang Chen a,∗, Lijuan Kang b,∗∗, Dadong Liang b,∗∗, Ruifa Jin d a
College of Life Sciences, Jilin Agricultural University, Changchun, Jilin 130118, China College of Resource and Environmental Science, Jilin Agricultural University, Changchun 130118, China Changchun Institute of Measurement and Testing Technology, Changchun, Jilin, China d College of Chemistry and Chemical Engineering, Chifeng University, Chifeng 024000, China b c
a r t i c l e
i n f o
Article history: Received 20 October 2016 Revised 20 March 2017 Accepted 10 April 2017 Available online xxx Keywords: Substituent effects Naphthalene diimide Acceptor Organic solar cells Theoretical study
a b s t r a c t The substituent effects of naphthalene diimide (NDI) have been investigated by employing the CAMB3LYP/6-31G(d) and TD-B3LYP/6-31+G(d,p) methods in order to design proper acceptor of solar cell with excellent performances, for example, the suitable frontier molecular orbital (FMO) energies to match those of oligo(thienylenevinylene) derivatives. The simulated results show that the different substituents significantly affect the distribution patterns of FMOs for NDI. The pull substituents could decrease the FMO energies and energy gap of NDI. Introducing the proper substituents to molecule NDI could make their FMOs being suitable for oligo(thienylenevinylene) derivatives. The different substituents significantly affect the absorption spectra of NDI. The electron withdrawing group and/or electron donating group substituents can improve the electron transfer properties of NDI. © 2017 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
1. Introduction Since the organic solar cells (OSCs) appeared, owing to its lightweight, mechanical flexibility, potential of low cost fabrication, and rapid energy payback time [1], it received worldwide attention in both academia and industry [2]. Now the OSCs mainly utilize fullerene and its derivatives, such as [6,6]-phenyl-C61 (or C71 )-butyric acid methyl ester (PC61 BM or PC71 BM), as the electron acceptor materials. Fullerene and its derivatives are good electron acceptors and electron transporting materials. Nevertheless, it is difficult to modify the backbone of fullerene and its derivatives chemically. Thus, the absorption region and the frontier molecular orbit (FMO) energy levels cannot be readily tuned [3]. Recently, more and more researchers pay attention to the organic small molecules on the basis of the π -conjugate system as the acceptor materials of the OSCs, because of their various molecular structures, easily tunable absorption region and FMO energy levels, low cost, and light weight [4]. Among these organic acceptors, naphthalene diimide (NDI) based on the conjugated polymer has ∗ Corresponding author at: College of Life Sciences, Jilin Agricultural University, Changchun, Jilin 130118, China. ∗∗ Corresponding authors. E-mail addresses: [email protected], [email protected] (G. Chen), [email protected] (D. Liang).
been recognized as the most effective acceptor material [5–7]. Zhou et al. investigated the active layer photovoltaic performance, charge transport, and blend morphology of the all-polymer OSCs construct of p-type PTPD3T and n-type N2200 (NDI derivative) [8]. Their work not only could highlight the importance of molecular weight tuning for both polymer components, but also could provide a promising strategy and relevant synthetic methodologies to utilize the conjugated polymers with desired molecular weight for optimizing future efficiencies of all-polymer solar cells. Zhou et al. fabricated the OSCs on the basis of the copolymer of NDI and bithiophene as the near-infrared absorber and electron acceptor, and PTB7 was as the electron donor [9]. The external quantum efficiency spectra of the OSCs displayed photoresponse up to 900 nm with the efficiency of 25% at 800 nm. Earmme et al. provided an investigation of three NDI copolymers as electron acceptors in OSCs. They found that the highest power conversion efficiency (PCE) of OSCs on the basis of the NDI copolymer as acceptor and the thiazolothiazole copolymer as donor is 3.3% [10]. Its short circuit current density and external quantum efficiency are 7.78 mA/cm2 and 47%. Lee et al. produced a highly efficient OSC device with the PCE of 5.96% by taking a series of naphthalene diimide-based polymer as the acceptors [11]. Hwang et al. achieved an OSC device with the PCE of 7.7% utilizing the naphthalene diimide–selenophene copolymer as acceptor and
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O
R1
R2
H3C N O
Fig. 1. The structures of donors X1 and X2.
benzodithiophene–thieno[3,4-b]thiophene copolymer as donor [12]. Gao et al. prepared OSCs with a high PCE of 8.27% and a high fill factor of 70.24% using N2200 as acceptor and J51 as donor [13]. Hwang et al. designed a series of NDI/perylene diimide (PDI)–selenophene random copolymers, and investigated them as acceptors in OSCs [14]. They obtained the fine performances with the highest PCE of 6.3% and external quantum efficiency of 91%. Sakai et al. reviewed researches of the core-substituted naphthalene diimides [15] and investigated the electron- and hole-transporting pathways of naphthalene diimide derivatives [16]. Yushchenko et al. gave the comparison of charge-transfer dynamics of naphthalene diimide triads in solution [17]. Monti et al. investigated a series of donor–antenna–acceptor molecular rectifiers designed as modules for artificial photosynthesis devices with the naphthalene diimides as the antenna and secondary electron acceptor [18]. They also proposed a photoanode in which a TiO2 substrate is functionalized with the naphthalene diimide dye [19]. In the present work, we investigated a series of NDI derivatives with different substituent groups as OSCs acceptors. The purpose of introducing substituent groups is to improve the performances of NDI, such as the proper FMO energy levels matching those of the donors oligo(thienylenevinylene) derivatives (X1 and X2, Fig. 1) with excellent performances investigated by Yong and Zhang before [20], broad absorption region, and high charge transfer rate. Molecules X1 and X2 are the donor molecules in the study of Yong and Zhang with excellent properties, such as superior absorption properties. The difference between them is that the position of atom N in the substituent group connected with the thiophene is different. It is well known that, increasing the lowest unoccupied molecular orbital (LUMO) energy of the acceptor could increase the open circuit voltage (Voc ), because the Voc value increases along with the difference between the highest occupied molecular orbital (HOMO) energy of the donor and LUMO energy of the acceptor increasing. Additionally, the LUMO energy of the donor should be higher than that of acceptor larger than 0.30 eV The substituent groups could affect the molecular properties significantly, thus three kinds of molecules (NDI1-22, Scheme 1) were designed to study the push (–OCH3 and –C6 H6 ), pull (–CN and –NO2 ), and push–pull (–CN and –OCH3 as well as –C6 H6 and –NO2 ) substituent groups effects. We investigated the ground state properties of these molecules by using the density functional theory (DFT) [21], for example HOMO energy, LUMO energy, and HOMO–LUMO gap (Eg ). The absorption spectra of the designed molecules were evaluated by the time dependent DFT [22–24] approach (TD-DFT). We also simulated the charge transfer properties (reorganization energy, λ). Moreover, the correlation between structures and properties of the designed molecules were discussed.
O N CH3
R3
NDI1: R1 = -CN NDI2: R1 = R2 = -CN NDI3: R1 = R3 = -CN NDI4: R1 = R4 = -CN NDI5: R1 = -OCH3 NDI6: R1 = R2 = -OCH3 NDI7: R1 = R3 = -OCH3 NDI8: R1 = R4 = -OCH3 NDI9: R1 = -CN, R2 = -OCH3 NDI10: R1 = -CN, R3 = -OCH3 NDI11: R1 = -CN, R4 = -OCH3
R4
O
NDI12: R1 = -C6H6 NDI13: R1 = R2 = -C6H6 NDI14: R1 = R3 = -C6H6 NDI15: R1 = R4 = -C6H6 NDI16: R1 = -NO2 NDI17: R1 = R2 = -NO2 NDI18: R1 = R3 = -NO2 NDI19: R1 = R4 = -NO2 NDI20: R1 = -C6H6, R2 = -NO2 NDI21: R1 = -C6H6, R3 = -NO2 NDI22: R1 = -C6H6, R4 = -NO2
Scheme 1. Chemical structures of NDI derivatives. Table 1 The predicted EHOMO , ELUMO , and Eg values of NDI and its derivatives at the CAM-B3LYP/6-31G(d) level.
NDI NDI1 NDI2 NDI3 NDI4 NDI5 NDI6 NDI7 NDI8 NDI9 NDI10 NDI11 NDI12 NDI13 NDI14 NDI15 NDI16 NDI17 NDI18 NDI19 NDI20 NDI21 NDI22
EHOMO (eV)
ELUMO (eV)
Eg (eV)
−8.34 −8.74 −9.05 −9.12 −9.10 −7.90 −8.19 −7.71 −7.45 −8.60 −8.28 −8.24 −7.98 −7.70 −7.96 −7.83 −8.78 −9.13 −9.21 −9.18 −8.25 −8.31 −8.22
−2.28 −2.74 −3.09 −3.16 −3.18 −2.12 −2.11 −1.98 −1.99 −2.65 −2.57 −2.56 −2.22 −2.14 −2.15 −2.17 −2.70 −3.07 −3.11 −3.11 −2.57 −2.62 −2.63
6.07 6.00 5.96 5.96 5.92 5.77 6.09 5.73 5.46 5.95 5.71 5.68 5.76 5.56 5.81 5.66 6.08 6.06 6.10 6.07 5.68 5.70 5.60
2. Computational details Recently, DFT method attracts extensive attention in OSCs field worldwide because of its application in explanation and predicting the properties of OSCs [25,26]. The DFT method CAM-B3LYP/631G(d,p) was proved to be reasonable for optimization of perylene diimide (PDI) and its derivatives, and the TD-B3LYP/6-31+G(d,p) method reliable for optical property simulation by our previous works [27,28]. Moreover, the chemical structure of NDI (Scheme S1) and the calculated results (Table S1) are listed in the supporting information. It could be seen that the CAM-B3LYP method was reliable in comparison with the crystal data [29]. Consequently, we employed the CAM-B3LYP/6-31G(d,p) method to optimize all the geometry parameters including neutral, cation, and anion of molecules NDI1-22 with no symmetry. The B3LYP/6-31+G(d,p) method was used to predict the absorption spectra of molecules NDI1-22. There is no imaginary frequency in the calculations. The PBE1PBE/6-31G(d) method was used to optimize the geometry of molecules X1 and X2 [20], and the HOMO and LUMO energies of molecules X1 and X2 were calculated at the CAM-B3LYP/6-
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31G(d,p) level on the basis of the single point energy. The CAMB3LYP/6-31G(d,p) single point calculations were performed according to the CAM-B3LYP/6-31G(d,p) optimized neutral, cationic, and anionic geometries. The reorganization energy is composed of two parts, external reorganization energy (λext ) and internal reorganization energy (λint ). λext reflects the effect of polarized medium on
3
charge transfer. λint represents the structural change between ionic and neutral states [30]. For the solid-state film of OSC materials, its low dielectric constant of medium leads to λext being not the main factor of λ [31]. Hence, we mainly discussed the λint of the isolated active organic π -conjugated systems owning to ignoring any environmental relaxation and changes in the present work. As
Fig. 2. The distribution patterns of FMO for NDI and its derivatives.
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Fig. 2. Continued
Fig. 3. The EHOMO and ELUMO values of FMOs for NDI and its derivatives.
where E0 + (E0 − ) means the energy of the cation (anion) obtained by optimizing structure of the neutral molecule. Similarly, E+ (E− ) represents the energy of the cation (anion) obtained on the basis of the optimized cation (anion) structure, and E+ 0 (E− 0 ) is the single point energy of the neutral molecule simulated at the cationic (anionic) state. Finally, E0 is the single point energy of the neutral molecule calculated at the ground state. All the calculations were performed with the aid of Gaussian 09 software [32]. 3. Results and discussion 3.1. Frontier molecular orbitals
Fig. 4. The simulated HOMO and LUMO energies of NDI2-4 and NDI17-19 at the CAM-B3LYP/6-31G(d,p) level as well as the HOMO and LUMO energies of X1 and X2 at the CAM-B3LYP/6-31G(d,p)//PBE1PBE/6-31G(d) level.
a result, Eqs. (1) and (2) can be used for simulating the electron reorganization energy (λe ) and hole reorganization energy (λh ) values [31].
λe = E0 − − E− + E− 0 − E0
(1)
λh = E0 + − E+ + E+ 0 − E0
(2)
The distribution patterns of the FMO are important for characterizing the optical and electronic properties of molecules. The electron spatial distributions of HOMO and LUMO orbitals of NDI and its derivatives are plotted in Fig. 2. As shown in Fig. 2, for molecules NDI and NDI1-11, the distribution patterns of FMOs are spread over the whole molecules. It reveals that the –CN and –OCH3 groups with different amount and positions affect the FMO distributions of NDI slightly. For –C6 H6 group substituent molecules, the HOMO patterns of NDI12 and NDI15 are distributed on the NDI molecules and the –C6 H6 groups. Their LUMO patterns are delocalized on the NDI molecules. For molecules NDI13 and NDI14, their HOMO electronic density contours are distributed on their –C6 H6 groups, and their LUMO electronic density contours delocalized on the NDI molecules. These results show that the amount and positions of –C6 H6 groups affect the electronic density contours of FMOs for NDI significantly. For
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Table 2 Calculated R (nm), λmax (nm), λmin (nm), and corresponding f values of NDI and its derivatives (H = HOMO, L = LUMO, L + 1 = LUMO + 1, etc.).
λmax
f
Composition
λmin
f
Composition
R
NDI
366.65
0.30
H → L (+70%)
219.71
0.06
146.94
NDI1
371.45
0.25
H → L (+70%)
233.28
0.08
NDI2
377.56
0.20
H → L (+70%)
252.41
0.13
NDI3
374.02
0.21
H → L (+70%)
246.99
0.24
NDI4
376.11
0.21
H → L (+70%)
255.69
0.56
NDI5
403.66
0.20
H → L (+69%)
228.03
0.06
NDI6
363.88
0.26
H-2 → L (−18%) H → L (+67%)
230.63
0.02
NDI7
406.31
0.20
H → L (+70%)
238.54
0.04
NDI8 NDI9
435.71 395.18
0.21 0.01
H → L (+70%) H-3 → L (+61%) H-2 → L (−21%) H-1 → L (+11%) H → L (−22%)
225.85 238.73
0.17 0.01
NDI10
413.54
0.18
H → L (+69%)
236.31
0.04
NDI11
411.72
0.19
H → L (+69%)
240.53
0.02
NDI12
468.78
0.04
H → L (+69%)
236.48
0.16
NDI13
363.62
0.27
H-4 → L (+70%)
306.51
0.17
NDI14
363.60
0.27
H-4 → L (+70%)
305.66
0.17
NDI15
474.29
0.08
H → L (+69%)
269.78
0.33
NDI16
367.88
0.23
266.44
0.02
NDI17
379.01
0.01
H-2 → L (+12%) H → L (+69%) H-4 → L (+24%) H-3 → L (+62%) H → L (−20%)
271.86
0.02
H-4 → L + 1 (+69%) H-2 → L + 1 (−14%) H-5 → L + 1 (−33%) H-4 → L + 1 (+42%) H-2 → L + 1 (+31%) H-2 → L + 2 (−27%) H → L + 1 (+10%) H-8 → L (+23%) H-5 → L + 1 (−15%) H-5 → L + 3 (+10%) H-2 → L + 1 (+46%) H → L + 2 (+42%) H-9 → L (−12%) H-8 → L (−23%) H-4 → L + 3 (−16%) H-2 → L (+12%) H → L + 1 (−14%) H → L + 2 (+58%) H-5 → L (−19%) H-5 → L + 1 (−11%) H-5 → L + 3 (+13%) H-3 → L (+16%) H-3 → L + 1 (−11%) H → L + 1 (+61%) H-3 → L + 1 (+67%) H → L + 3 (−16%) H-11 → L (−14%) H-5 → L + 1 (+11%) H-7 → L + 1 (−19%) H-5 → L + 1 (−17%) H-4 → L + 3 (−11%) H-2 → L + 2 (−35%) H-2 → L + 3 (+27%) H → L + 3 (+37%) H-3 → L + 1 (+65%) H → L + 2 (+11%) H → L + 3 (−21%) H-4 → L + 1 (+68%) H-7 → L + 1 (+10%) H-6 → L + 1 (+12%) H-4 → L + 1 (+20%) H-3 → L + 2 (+54%) H-2 → L + 1 (+14%) H-2 → L + 2 (−26%) H-1 → L + 3 (+12%) H-3 → L + 1 (+65%) H-2 → L + 1 (−15%) H-2 → L + 2 (+12%) H-3 → L + 1 (+65%) H-2 → L + 2 (−19%) H-11 → L (+12%) H-5 → L + 1 (−20%) H-2 → L + 2 (+47%) H-2 → L + 3 (+33%) H-1 → L + 2 (−12%) H-1 → L + 3 (−12%) H-9 → L (+68%) H-4 → L+2 (−18%) H-9 → L (+66%) H-7 → L (+14%) H-4 → L + 2 (−18%) H-1 → L + 3 (+17%) H → L + 2 (+65%) H-11 → L (+49%) H → L + 2 (−48%) H-13 → L (+16%) H-12 → L + 1 (−25%) H-7 → L + 2 (−25%) H-7 → L + 3 (+14%) H-6 → L + 1 (−11%) H-5 → L + 2 (+11%) H-3 → L + 1 (+13%) H-1 → L + 2 (−24%) H-1 → L + 3 (+12%) H → L + 1 (+38%)
138.17
125.15
127.03
120.42
175.63 133.25
167.77
209.86 156.45
177.23
171.19 232.30
57.11 57.94
204.51 101.44 107.15
(continued on next page)
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S. Tang et al. / Journal of the Taiwan Institute of Chemical Engineers 000 (2017) 1–9 Table 2 (continued)
λmax
f
Composition
λmin
f
Composition
R
NDI18
366.53
0.20
297.94
0.07
H-7 → L (+70%)
68.59
NDI19
375.83
0.14
H-1 → L (−13%) H → L (+68%) H-1 → L (+25%) H → L (+65%)
263.21
0.05
112.62
NDI20
368.74
0.22
320.43
0.13
NDI21
366.49
0.22
H-3 → L H-2 → L H-3 → L H-2 → L
279.91
0.01
NDI22
491.88
0.04
276.12
0.12
H-14 → L (+60%) H-1 → L + 4 (−11%) H → L + 4 (+32%) H-7 → L (+69%) H-2 → L + 3 (+13%) H-11 → L (+64%) H-10 → L + 1 (−10%) H-2 → L + 2 (+14%) H-2 → L + 3 (−16%) H-11 → L (+30%) H-2 → L + 3 (−13%) H → L + 3 (+56%) H → L + 4 (+19%)
(+17%) (+68%) (+16%) (+68%)
H → L (+69%)
molecules NDI6-19, the distribution patterns of FMOs are spread over the whole molecules. It displays that the –NO2 groups with different amount and positions affect the FMO distributions of NDI slightly. For molecules NDI20 and NDI21, their HOMO electronic density contours are distributed on the –C6 H6 groups, and their LUMO electronic density contours delocalized on the –NO2 groups and NDI molecules. For molecule NDI21, its HOMO pattern is nearly spread over the whole molecule, and its LUMO pattern is distributed on the –NO2 group and NDI molecule. In conclusion, the –CN and –OCH3 substituents affect the FMO patterns of NDI molecule slightly, and the –C6 H6 and –NO2 substituents affect them significantly. As is well-known, the FMO energies are very important for the spectral properties of the designed molecules, and they also relate to the reactivity of a compound. The evaluations of HOMO and LUMO energies (EHOMO and ELUMO ) for the investigated molecules are listed in Table 1 and plotted in Fig. 3. As shown in Table 1 and Fig. 3, one can find that introducing the –CN group to molecule NDI could decrease the EHOMO , ELUMO , and Eg values of NDI. The amount of decrease for EHOMO and ELUMO values are in the order of NDI4 > NDI3 > NDI2 > NDI1. Their Eg values are in the sequence of NDI4 < NDI3 = NDI2 < NDI1 < NDI. It reveals that –CN substituent can decrease the EHOMO , ELUMO , and Eg values of NDI, and di-CN substituents make larger decrease of EHOMO , ELUMO , and Eg values of NDI than the single –CN substituent does. For molecules NDI5-8, the –OCH3 group could increase the EHOMO and ELUMO values and decrease the Eg values of NDI, except the –OCH3 groups in 1,2-positions increase the Eg value of NDI. The increase of the EHOMO value is the largest when the –OCH3 group in the 4-position of NDI. The increase of the ELUMO value is the largest when the –OCH3 group is in the 3-position of NDI. The Eg value is the smallest when the –OCH3 group in the 4-position of NDI. For –CN and –OCH3 substituent molecules, their EHOMO values are in the order of NDI11 > NDI10 > NDI > NDI9, ELUMO values are in the sequence of NDI11 > NDI10 > NDI9 > NDI, and Eg values are in the order of NDI11 < NDI10 < NDI9 < NDI. These results display that the push–pull substituents influence the EHOMO , ELUMO , and Eg values of NDI significantly, and decrease the Eg values of NDI. For molecules NDI12-15, the –C6 H6 group could increase the EHOMO and ELUMO values and decrease the Eg values of NDI. The increases of the EHOMO and ELUMO values are the largest when the –C6 H6 group is in the 2-position of NDI, respectively. The Eg value is the smallest when the –C6 H6 group is in the 4-position of NDI. For –NO2 substituent molecules, their EHOMO values are in the order of NDI > NDI16 > NDI17 > NDI19 > NDI18, ELUMO values are in the sequence of NDI > NDI16 > NDI17 > NDI18 = NDI19, and Eg values are in the order of NDI18 > NDI16 > NDI = NDI19 > NDI17. These results indicate that –NO2 substituent could
48.31 86.58
215.76
decrease the EHOMO and ELUMO values of NDI obviously, and affect the Eg values of NDI slightly. For molecules NDI20-22, the –C6 H6 and –NO2 substituents could influence the EHOMO values slightly, and decrease the ELUMO and Eg values obviously. In conclusion, the pull substituents could decrease the EHOMO , ELUMO , and Eg values of NDI. The EHOMO and ELUMO values of molecules X1, X2, NDI2-4, and NDI17-19 are plotted in the Fig. 4. From Fig. 4, one can see that the ELUMO values of NDI2-4 and NDI17-19 are lower than those of X1 (0.41, 0.48, 0.50, 0.39, 0.43, and 0.43 eV) and X2 (0.39, 0.46, 0.48, 0.37, 0.41, and 0.41 eV), which indicates that NDI2-4 and NDI1719 are suitable acceptors for the FMOs of X1 and X2, respectively. These results show that the di-CN and di-NO2 sbstituents can decrease the FMOs of NDI. Therefore, introducing the proper substituents to molecule NDI could make their FMOs being suitable for molecules X1 and X2. 3.2. Absorption spectra The absorption region (R), the longest wavelengths of the absorption spectra (λmax ), the shortest wavelengths of the absorption spectra (λmin ), and the corresponding oscillator strength (f) of designed molecules are listed in Table 2. The simulated absorption spectra of designed molecules (considering the first 20 excited states) are shown in Fig. 5. The simulated absorption spectra of designed molecules were plotted by using the GaussSum 1.0 program [33]. As shown in Table 2 and Fig. 5, for molecules NDI1-5, the –CN substituent in different positions could increase the λabs-max and λabs-min values, and decrease the R values of NDI, respectively. Moreover, the amounts of R values decreased for di-CN substituent molecules are larger than that of single –CN substituent molecule. The calculated λabs-max value of molecule NDI4 is 376.11 nm, and the experiment datum of this is 380 nm [15]. It reveals that the computational methods are reasonable. For –OCH3 substituent molecules, the λabs-max values are in the increasing order NDI6 < NDI < NDI5 < NDI7 < NDI8, the λabs-min values are in the decreasing sequence NDI > NDI8 > NDI5 > NDI6 > NDI7, and the R values are in the order NDI6 < NDI < NDI7 < NDI5 < NDI8. It shows that the –OCH3 substituent can increase the λabs-max , λabs-min , and R values of NDI, except the diOCH3 substituent in 1,2-positions decrease the λabs-max and R values of NDI. In addition, the di-OCH3 substituent in 1,4-positions could make larger increase of R value than the other positions for NDI. For molecules NDI9-11, the –CN and –OCH3 substituents can increase the λabs-max , λabs-min , and R values of molecule NDI, and the di-CN–OCH3 substituents in 1,3-positions could make the largest increase of R value for NDI. For –C6 H6 substituent
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Fig. 5. The calculated absorption spectra of NDI and its derivatives (value of full width at half maximum is 30 0 0 cm−1 ).
molecules, the λabs-max values are in the order NDI15 > NDI12 > NDI > NDI13 ≈ NDI14, the λabs-min values are in the sequence NDI < NDI12 < NDI15 < NDI13 ≈ NDI14, and the R values are in the order NDI13 ≈ NDI14 < NDI < NDI15 < NDI12. It indicates that the –C6 H6 substituent in 1-position and 1,4-positions can increase
the R value, and the –C6 H6 substituent in 1,2-positions and 1,3positions can decrease the R value of molecule NDI. For molecules NDI16-19, the –NO2 substituent affects the λabs-max values slightly, increases the λabs-min values obviously, and decreases the R values significantly of molecule NDI. For –C6 H6 and –NO2 substituent
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Fig. 5. Continued
molecules, the λabs-max values are in the order NDI ≈ NDI21 < NDI20 < NDI22, the λabs-min values are in the decreasing sequence NDI20 > NDI21 > NDI22 > NDI, and the R values are in the increase order NDI20 < NDI21 < NDI < NDI22. It reveals that –C6 H6 and –NO2 substituents in 1,2- and 1,3-positions can affect the λabs-max values slightly, increase the λabs-min values obviously, and decrease the R values significantly of molecule NDI. The –C6 H6 and –NO2 substituents in 1,4-positions can increase the λabs-max , λabs-min , and R values significantly of molecule NDI. In conclusion, the λmax value of NDI22 is the largest and the R value of NDI12 is the largest among the designed molecules, indicating that they could be good potential candidates for the solar cell acceptors. 3.3. Charge transport property The charge transport property of material is important to design the acceptor for solar cell device, and the λ plays a role in charge transport and charge separation [34–39]. Understanding the relationship between molecular structure and charge transport property is important for designing good acceptors of solar cell devices. It is well known that the lower the reorganization energy, the higher the charge transfer rate [40,41]. We calculated the λ values of designed molecules on the basis of Eqs. (1) and (2). The calculated results are summarized in Table 3. As shown in Table 3, for molecules NDI1-4, the –CN substituent could decrease the λe values and increase the λh values of molecule NDI, except the –CN substituent in 1,3-positions could decrease the λh value of molecule NDI. It reveals that –CN substituent could improve the electron transfer rate of molecule NDI. For molecules NDI5-8, their λe and λh values are larger than those of molecule NDI, respectively. It indicates that –OCH3 substituent could decrease the charger transfer rate of molecule NDI. The λe values of molecules NDI9-11 are smaller than that of molecule NDI, and their λh values are larger than that of molecule NDI. It shows that –CN and –OCH3 substituents could increase the electron transfer property and decrease the hole transfer property of molecule NDI. For molecules NDI12-15, their λe and λh values are larger than those of molecule NDI, respectively, except the λe values of NDI12, NDI13, and NDI13 are smaller than that of molecule NDI. It displays that –C6 H6 substituent in 1-, 1,2- and 1,4-positions could increase the electron transfer rate of NDI. For molecules NDI16-22, their λe and λh values are larger than those of molecule NDI, respectively. It indicates that –NO2 as well as
Table 3 Calculated λe and λh (eV) values of NDI and its derivatives. CAM-B3LYP/6-31G(d,p)
NDI NDI1 NDI2 NDI3 NDI4 NDI5 NDI6 NDI7 NDI8 NDI9 NDI10 NDI11 NDI12 NDI13 NDI14 NDI15 NDI16 NDI17 NDI18 NDI19 NDI20 NDI21 NDI22
λe
λh
0.435 0.403 0.387 0.376 0.367 0.457 0.502 0.476 1.853 0.433 0.421 0.428 0.426 0.428 0.447 0.417 0.445 0.454 0.448 0.448 0.437 0.444 0.455
0.216 1.118 0.215 0.283 0.217 0.394 0.344 0.619 1.870 0.792 0.391 0.374 1.217 0.577 1.197 0.528 0.286 0.302 0.285 0.291 1.273 1.067 1.018
–C6 H6 and –NO2 substituents could decrease the charger transfer rate of molecule NDI. In conclusion, among these molecules, NDI4 has the best electron transport property, and NDI2 has the best hole transport property. 4. Conclusion In this work, we simulated the properties of the NDI and its derivatives. The calculated results revealed that the –CN and –OCH3 substituents affect the FMO patterns of NDI molecule slightly, and the –C6 H6 and –NO2 substituents affect them significantly. The –CN substituent can decrease the EHOMO , ELUMO , and Eg values of NDI. The –OCH3 group could increase the EHOMO and ELUMO values. The –C6 H6 group could increase the EHOMO and ELUMO values and decrease the Eg values of NDI. The –NO2 substituent could decrease the EHOMO and ELUMO values of NDI obviously, and affect the Eg values of NDI slightly. The different substituents in different positions could affect the λabs-max ,
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λabs-min , and R values significantly. The λmax value of NDI22 is the largest and the R value of NDI12 is the largest among the designed molecules. The –CN substituent could improve the electron transfer rate of molecule NDI. The –OCH3 substituent could decrease the charger transfer rate of molecule NDI. The –NO2 substituent could decrease the charger transfer rate of molecule NDI. Among these molecules, NDI4 has the best electron transport property, and NDI2 has the best hole transport property. Molecules NDI24 and NDI17-19 are suitable acceptors for X1 and X2, respectively. This study should be helpful in further theoretical and experimental study of such system on solar cell acceptor materials. Acknowledgments This work was financially supported by the Science and Technology Development Program Project of Jilin Province (No. 20170520145JH), the High-end Technology Innovation Platform of Straw Comprehensive Utilization Technology of College and University in Jilin Province (No. (2014)C-1), the Scientific Research Plan Project of the Education Department of Jilin Province (No. 2016170), and the National Natural Science Foundation of China (Nos. 21302062 and 21563002). Supplementary materials Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.jtice.2017.04.015. References [1] Darling SB, You F. The case for organic photovoltaics. RSC Adv 2013;3:17633–48. [2] Pelzera KM, Darling SB. Charge generation in organic photovoltaics: a review of theory and computation. Mol Syst Des Eng 2016;1:10–24. [3] He Y, Li Y. Fullerene derivative acceptors for high performance polymer solar cells. Phys Chem Chem Phys 2011;13:1970–83. [4] Pelzera KM, Darling SB. Charge generation in organic photovoltaics: a review of theory and computation. Mol Syst Des Eng 2016;1(1):10–24. [5] Zhan C, Yao J. More than conformational “twisting” or “coplanarity”: molecular strategies for designing high-efficiency nonfullerene organic solar cells. Chem Mater 2016;28:1948–64. [6] Tang XQ , Liu XR, Shen W, Hu WX, He RX, Li M. Theoretical investigations of the small molecular acceptor materials based on oligothiophene-naphthalene diimide in organic solar cells. RSC Adv 2016;6:102159–71. [7] Xue LW, Yang YK, Zhang ZG, Dong XN, Gao L, Bin HJ, et al. Indacenodithienothiophene-naphthalene diimide copolymer as an acceptor for all-polymer solar cells. J Mater Chem A 2016;4:5810–16. [8] Zhou NJ, Dudnik AS, Li TING, Manley EF, Aldrich TJ, Guo PJ, et al. All-polymer solar cell performance optimized via systematic molecular weight tuning of both donor and acceptor polymers. J Am Chem Soc 2016;138:1240–51. [9] Zhou EJ, Nakano M, Izawa S, Cong JZ, Osaka I, Takimiya K, et al. All-polymer solar cell with high near-infrared response based on a naphthodithiophene diimide (NDTI) copolymer. ACS Macro Lett 2014;3:872–5. [10] Earmme T, Hwang YJ, Murari NM, Subramaniyan S, Jenekhe SA. All-polymer solar cells with 3.3% efficiency based on naphthalene diimide-selenophene copolymer acceptor. J Am Chem Soc 2013;135:14960–3. [11] Lee C, Kang H, Lee W, Kim T, Kim KH, Woo HY, et al. High-performance all-polymer solar cells via side-chain engineering of the polymer acceptor: the importance of the polymer packing structure and the nanoscale blend morphology. Adv Mater 2015;27:2466–71. [12] Hwang YJ, Courtright BAE, Ferreira AS, Tolbert SH, Jenekhe SA. 7.7% efficient all-polymer solar cells. Adv Mater 2015;27:4578–84. [13] Gao L, Zhang ZG, Xue L, Min J, Zhang J, Wei Z. All-polymer solar cells based on absorption-complementary polymer donor and acceptor with high power conversion efficiency of 8.27%. Adv Mater 2016;28:1884–90. [14] Hwang YJ, Earmme T, Courtright BAE, Eberle FN, Jenekhe SA. n-type semiconducting naphthalene diimide-perylene diimide copolymers: controlling crystallinity, blend morphology, and compatibility toward high-performance all-polymer solar cells. J Am Chem Soc 2015;137:4424–34. [15] Sakai N, Mareda J, Vauthey E, Matile S. Core-substituted naphthalenediimides. Chem Commun 2010;46:4225–37.
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Please cite this article as: S. Tang et al., The substituent effects of naphthalene diimide as acceptor for organic solar cells: A theoretical study, Journal of the Taiwan Institute of Chemical Engineers (2017), http://dx.doi.org/10.1016/j.jtice.2017.04.015